Could a little dashboard light cost you big bucks?

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When the temperature drops, so does the air pressure in your tires, and that can cause a warning light to pop up on your dashboard. It's completely natural, and the fix is easy: Just add some air. But could it scare you into spending big money that you don't have to spend? To find out, TODAY launched a hidden-camera investigation — with surprising results.

The van was then wired with five hidden cameras: underneath, inside the side-view mirror, even behind the tire, capturing every angle. Then a female TODAY producer was sent to six shops around New Jersey, from car dealers to national chains to independent garages, with that tire-pressure warning light on.

Things got off to a promising start at two Chrysler dealers. "Just go to a gas station, put air in it, drive, and that light will go out," advised one. "That's it." The dealers didn't charge anything.

Next, at a Midas shop, the mechanic said, "You don't need to fix anything. It's just low on air."

Things got even better at a Firestone shop, where they even took the tire off to inspect it for leaks. The service was thorough and honest — and free.

After being informed about the hidden cameras, the Firestone owner explained: "You've got to trust your auto mechanic like you've got to trust your doctor. You've got to go somewhere where you can trust the person."

But what happened at an independent shop was the most surprising of all. The TODAY producer told them she was clueless, making her an easy target: "I just don't know anything about cars."

"Don't worry about that," the owner told her, and advised: "Don't tell the mechanic you don't know anything. He's going to rip you off. We are not like that, so I'll give you a tip ... You don't need tires. We change it, we make money, but if you don't need it, you don't need it."

At a Firestone shop, the service was thorough, honest and free.Today

After being told he'd been on hidden camera, the owner explained: "That's the way I was brought up: Be honest and help people as much as you can."

In the end, every single repair shop TODAY visited did the right thing: a Rossen Reports first. Even expert Audra Fordin was stunned.

"To see that they were really trying to give great service, helping out someone who really just didn't know, was really wonderful," she said. These shops are proof that you can have a thriving business by being honest and giving good service.

What should you do if you wake up one morning and your tire pressure warning light is on in your car? You can fix it yourself, in just a few minutes, and this simple how-to video will show you how: